Heat zones indicate the average number of days each year when temperatures exceed 86°F, helping gardeners understand how much heat plants can tolerate during the growing season. While cold-hardiness zones focus on minimum winter temperatures, heat zones address the impact of prolonged summer warmth, which can be just as limiting to plant performance. This is especially useful when selecting plants that may survive winter but struggle with high summer temperatures. By considering both cold hardiness and heat tolerance, gardeners can make better plant choices, reduce plant stress, and improve overall success in the landscape.
In 1997, the American Horticulture Society (AHS) developed the AHS Plant Heat Zone Map. This map has not been updated since. A new map from the Sustainable Gardening Library (SGI) is now available and provides the same information.
It divides the United States into zones based on the average number of days per year when daily high temperatures reach or exceed 86°F (30°C). This temperature was chosen because it is the point at which plants begin to suffer physiological damage from heat.
An interactive map is available here:
The Sustainable Gardening Institute Plant Heat Zones
More Information
- Sustainable Gardening Institute's Plant Heat Zone Interactive Map
- Sustainable Gardening Library's EPA EcoRegions Interactive Map
- Gardening in Iowa Zones
- Climate Zones from NOAA
- 2023 USDA Plant Hardiness Zone Map
- EcoRegions from EPA
Source: Sustainable Gardening Library